1. Field of the Invention
The present invention relates to a liquid jet recording head and more particularly it relates to a liquid jet recording head which discharges a recording liquid as liquid droplets and which can make a gradation record.
2. Related Background Art
Hitherto, non-impact recording methods have attracted attention because they produce little noise. Especially, the liquid jet recording method (ink-jet recording method) is a very useful method which makes a high-speed recording possible and which, besides, makes it possible to record on normal paper without the special treatment of fixation. Thus, many proposals have been made for various systems using such method and apparatuses for practicing them and some of them have been further improved and commercialized. Until now, efforts have been made for practical use of these methods.
Above all, those which are disclosed in Japanese Patent Application Laid-Open No. 51837/1979 and West German Laid-Open Application (DOLS) No. 2843064 have characteristics different from other ink-jet recording systems in that heat energy is allowed to act on a liquid to obtain power to discharge a recording liquid as liquid droplets.
That is, according to the recording systems disclosed in the above publications, the liquid which has undergone the action of heat energy changes in its state with an abrupt increase in volume, which includes generation of bubbles, and action based on said change in state permits the recording liquid to be discharged as droplets from orifices of the tip portion of the recording head and these droplets adhere to a recording member to make a record.
Furthermore, the ink-jet recording system disclosed in DOLS 2843064 has the advantage that images of high resolution and high quality can be obtained at high speed because the recording head part can easily be formed as a high density multi-orifice device of full-line type.
While, as explained above, liquid jet recording apparatuses have many advantages, in order to record images of higher resolution and higher quality, it has been required to give gradation to the picture elements to record images containing halftime information.
Hitherto, as systems for providing such liquid jet recording apparatus with gradation controllability, there have been known a first system, (1) according to which one picture element is composed of plural cells arranged in a matrix form and gradation of the desired level is digitally expressed depending on the number of cells and state of arrangement of these cells which are occupied by image forming elements realized in the cells arranged in matrix form, and a second system (2) according to which one picture element is formed of respective image forming elements and the desired gradation is analoguely expressed by changing optical density of the image forming elements.
However, in the case of the liquid jet recording methods which records by discharging liquid by heat energy, according to the above (1) gradation control system (the first system), the area of one picture element per se increases, which results in a reduction of resolution, etc. Furthermore, because of digital control, steps of gradation are large and sometimes the image obtained lacks fineness in texture. On the other hand, according to the above gradation control system (2) (the second system), in general, the size of one picture element, namely, the size of the image forming element, may be changed by changing electrical energy applied to an energy generator and in this case, sometimes, sufficient gradation control cannot be obtained.
Therefore, as disclosed, for example, in Japanese Patent Application Laid-Open No. 132259/1980, there has been proposed a recording head wherein plural heater elements are arranged in line with the discharge direction in the nozzle and the number of operating heater elements is controlled to change the size of the heat acting area, whereby modulation of volume of bubbles is effected by variation of area in which the bubbles are generated.
Moreover, according to the recording head disclosed in U.S. Pat. No. 4,251,824, at least two heating elements different in area of heater are arranged in the discharging direction in a nozzle and one suitable heater is selected in accordance with input signal to make dot diameter changeable, thereby to control gradation.
That is, in the case of the above-mentioned recording heads, plural heating elements are arranged in along the liquid supply direction in a nozzle and the heat acting area is changed by selection of these heater elements or operation of plural heating elements in combination, whereby dot diameter is changed to control gradation.
However, when plural heating elements are arranged in the liquid supply direction in the nozzle as mentioned above, the relative distance between said heating elements and discharge opening of nozzle is varied.
Especially when the entering direction of ink into the heat acting part and the discharging direction of the ink from the heat acting part are different as disclosed in U.S. Pat. No. 4,330,787 and 4,459,600, that is, when the discharge openings are provided at a face opposite to the heat acting face, and when relative positional relation between the center of bubble generation, namely, the center of the heat acting part, and the discharge opening changes as a result of using the abovestated construction, sometimes, there occurs deviation in the discharge direction of the ink. Furthermore, in some cases, such recording head is not suitable for highspeed recording due to change of discharging characteristics. Especially when the number of the heating elements increases, the above-mentioned tendency becomes conspicuous and so hitherto, area or the number of the heating elements has been subject to those limitations.